The cultivation of bulbous and tuberous plants is a difficult process. Mostly after harvesting the bulbs or tubers are stored for prolonged periods of several months before they are planted in the earth again. After re-planting mostly it takes several weeks up to several months before the plant develops. Furthermore, most of these crops are grown in the open air, where all kind of negative influences determine the development of the crop, e.g., moulds, insects, parasites and weather conditions such as a too high humidity. Also during storage and transport the quality of the bulbs or tubers can be affected in a negative way. To avoid deterioration of the bulbs or tubers, they are mostly stored under more or less controlled environmental conditions.
Although bulbs and tubers are subjected to various biotic and abiotic threats, development of moulds during storage and after planting can be considered as one of the main problems. Only in The Netherlands the economic losses due to mouldiness of flower bulbs is estimated at  200 million per year, this in spite of the extensive use of synthetic fungicides.
On flower bulbs of which tulip and lily are the most important crops Fusarium (e.g., Fusarium oxysporum) and Penicillium species are known to cause most of the problems. However, on these and other bulbs also other moulds such as Botrytis species, Stagnospora species, Rhizoctonia species and Pythium species may occur. On seed-potatoes Fusarium species (e.g., Fusarium solani), Rhizoctonia solani, Helminthosporium solani, Phoma species and Penicillium species are examples of well known pathogenic moulds.
Since due to upcoming EU regulation it is expected that some of the frequently used synthetic fungicides will be banned, the future problems concerning moulds are expected to become even worse. To prevent considerable economic losses for the industry in the near future, there is a need for environmental friendly natural fungicides. Also from an environmental and health point of view it is of importance to obtain alternatives for the harmful synthetic fungicides which are applied nowadays.
For many decades the polyene macrolide antimycotic natamycin has been used to prevent fungal growth on food products such as cheeses and sausages. This natural preservative, which is produced by fermentation using Streptomyces natalensis, is widely used throughout the world as a food preservative and has a long history of safe use in the food industry. It is very effective against all known food spoilage moulds. Although natamycin is applied for many years in e.g. the cheese industry up to now development of resistant mould species was never observed.
Cheeses and sausages are treated by immersion or by spraying with a suspension of natamycin in water. Cheeses can also be covered by an emulsion of a plastic coating of mostly polyvinyl acetate in water containing natamycin. Casings applied on sausages can be soaked in a saturated suspension of natamycin. In case of beverages such as fruit juices, natamycin is simply dissolved in the end product.
Natamycin has a low solubility in water of 30-50 ppm; only the dissolved fraction has antifungal activity. Since natamycin has a MIC (Minimal Inhibitory Concentration) of less than 10 ppm for most fungi, the dissolved concentration is in most cases sufficient to prevent mould development. Under normal hygienic conditions denaturation of dissolved natamycin is compensated by dissolution of natamycin from the crystals and diffusion over the surface to the site of contamination.
Already in the 1970ties it was realized that natamycin (pimaricin) might be of value for control of fungal plant diseases on flower bulbs (see Dekker and Langerak (1979)). It was demonstrated that natamycin was as effective as organic mercury or formalin in preventing the spread of Fusarium oxysporum in the water baths in which the bulbs were dipped for two hours at 43.5° C. to eliminate nematodes, insects and mites. Natamycin effectively eliminated the Fusarium mould preventing cross contamination from diseased to healthy bulbs in the water bath.
However, in spite of these positive results of thirty years ago and the observation that under laboratory conditions natamycin was effective in combating fungal species occurring on crops such as flower bulbs, tubers and seed-potatoes, in practice treatment of these crops with even high natamycin concentrations was not effective in preventing mould development. Therefore, up to now this environmental friendly antifungal agent was never applied in practise on e.g. flower bulbs and comparable mould-sensitive crops such as tubers, onions and seed-potatoes.
In patent application WO 2004/067699 it is disclosed that a composition containing lignosulphonates together with a wide selection of other compounds can protect agricultural crops against threats such as weeds, biotic and abiotic stresses, insects, nematodes and pathogenic micro-organisms such as moulds, bacteria and viruses. Polyphenols and especially lignosulphonates are applied to enhance the effectiveness of other active compounds such as pesticides, fungicides, herbicides and plant protection compounds. Natamycin is mentioned as an example of a suitable fungicide, while potassium phosphite is mentioned as an example of a plant protection compound. In Example 3 of WO 2004/067699 it is demonstrated that the combination natamycin-lignosulphonate was effective against Botrytis development on leaf tips. In Example 11 an experiment is described in which lignosulphonate and natamycin were used to protect tulips against the Fusarium mould. It is observed that the leaves growing from tulip bulbs treated with natamycin and lignosulphonate showed no yellow mould spots compared with the bulbs dipped in natamycin alone where some yellow spots were observed. However, in this experiment the quality of the bulbs was not reported.